Jul 6, 2015
A Salty Solution for Europa's Dark Streaks

Sen—Thinking about Jupiter's moon Europa might conjure images of a dirty snowball. The icy world has puzzled planetary astronomers since the first images of Europa were beamed back from the Voyager 1 and Voyager 2 spacecraft. They revealed a white sphere crisscrossed with dark brownish-red lines and grooves.

Europa was acquired by Voyager 2. Image credit: NASA/JPL

Europa is about the size of our own Moon and is thought to harbor an ocean twice as large as Earth's. The idea has been suggested perhaps these dark grooves and channels have something to do with the ocean water and slush underneath coming up to the surface perhaps through cracks and breaks in the ice crust. This slush then darkens over time as the newly formed surface ages. The origin of the parent material that composes the brownish cracks and what process is darkening it remains an open question.

New evidence suggests that the dark color of the lines could be produced by salt, NaCl, the same salt you likely have sitting in your kitchen cupboard.

Kevin Hand and Robert Carlson of NASA's Jet Propulsion Laboratory (JPL), in Pasadena California, set about to see if they could recreate these features in the lab by mimicking the conditions and radiation environment at Europa's surface or as Hand called it, making 'Europa in a can.'

They found that salt solutions could match the color seen in spacecraft images. The samples were exposed to radiation doses trying to recreate a surface sitting for 10 to 100 years. These experiments are run at higher levels of irradiation, which speed up the reaction time. There's always the question of whether that changes the chemical reaction or rates in some fundamental way, but it is the best you can do since we can't wait four gigayears for the answer. So for these experiments, one hour of laboratory irradiation of the sample at JPL was the equivalent amount of flux it would have received over a time span of 800 hours sitting on Europa’s surface. Hand found that the salt’s color changed from a white to yellow to a dark brown.

This result offers an origin of these dark streaks without having to involve more complicated deeper processes involving volcanism or deep-sea vents or sulfur delivered from Io. Other hypotheses have been put forward for the origin of the dark material, which suggested that it was sulfur and magnesium compounds darkened by irradiation. Ground-based telescopes have detected the signature of magnesium sulfate, but it is thought that this is delivered to the surface of Europa rather than coming from the interior. The volcanoes of neighboring Io are spewing sulfur and magnesium out into the surrounding area about Jupiter and likely some of it is sticking to the ice rather than being located in the cracks.

Europa's ocean would naturally be salty, and so based on Hand's scenario slush from the salty ocean reaching the surface would just be the dried salt from the subsurface ocean darkening naturally over time.

Irradiated sample of salt from the 'Europa in a can' expeirment from Hunt and Carlson. Image credit: NASA/JPL-Caltech

Both ESA and NASA are planning missions to the Jupiter system set to launch in the 2020s: NASA's dedicated Europa Clipper mission and ESA's JUpiter ICy moons Explorer (JUICE), set to study Jupiter and three of its largest moons.

JUICE will primarily focus on Jupiter and Ganymede but is set to have two targeted flybys of Europa while also taking other long distance measurements of the moon. NASA's Europa Clipper also won't orbit the icy satellite, but it will perform a whopping 45 close fly-bys to study its surface and interior in detail. To give some comparison, NASA's Galileo probe that orbited Jupiter back in the 1990s, only preformed 11 flybys of the icy moon. At the end of May, the suite instruments that will compose the science payload for the Europa Clipper were recently announced. The instruments selected include ice-penetrating radar to study the structure and thickness of the moon's icy shell and possible subsurface lakes. The spacecraft will also be armed with a compliment of cameras and spectrometers to probe the surface composition.

These new results if confirmed might provide a new way of dating the different terrains that compose the icy crust that will be studied in detail by the Europa Clipper and JUICE. In the lab, the darker the color, the longer the salt mixtures had been exposed to the radiation. For example, if the spacecraft detect a water plume and isolate its breakout point, the color of the streaks could determine if it's an old or a relatively young piece of crust.